Adult periodontitis is associated with elevated levels of several Gram negative organisms in the subgingival oral biofilm, including the asaccharolytic, obligate anaerobe Porphyromonas gingivalis. In this primary niche, P. gingivalis interacts with a variety of other Gram negative obligate and facultative anaerobes, such as Fusobacterium nucleatum, Treponema denticola, and Tannerella forsythus through specific receptor-ligand interactions. However, the initial colonization of the oral cavity by P. gingivalis likely occurs through adherence to organisms in the supragingival biofilm and the successful colonization of this niche by P. gingivalis is contingent upon a variety of factors such as reduced oxygen tension and sufficient nutritional sources. Consistent with this, P. gingivalis has been shown to also adhere to organisms in supragingival plaque that may provide it with physiologic support, such as Streptococcus gordonii and F. nucleatum. 